The present invention relates to a computer tomography system utilizing nuclear magnetic resonance phenomenon and more particularly to a magnetic resonance imaging (MRI) system effective to form an image of a brain surface structure at high speeds.
As a method of imaging the brain surface structure by using the MRI system, a surface anatomy scanning (SAS) method has been available. For reference, MR Imaging of Brain Surface Structures by K. Katada et al, Japanese Journal of Magnetic Resonance in Medicine, Vol. 9, No. 3 (1989) should be noted.
The SAS method takes advantage of the fact that cerebrospinal fluid (CSF) existent in the brain surface has a spin-spin relaxation time T2 longer than that of the other portion of the brain Specifically, the T2 is highly enhanced to obtain a signal from only the cerebrospinal fluid and the thus obtained signal is used for formation of an image of the brain surface structure. More particularly, the formation of an image of the brain surface structure is accomplished by utilizing the contrast between a signal obtained from a portion where the CSF layer is thick and a portion where the CSF comes into cerebral sulci and fissures and a signal obtained from a portion where the CSF layer is thin and a portion of convolution or gyrus.
For enhancement of the T2, the aforementioned SAS method uses the so-called long spin echo method. Accordingly, it takes a long time to complete measurement. For example, given that the pulse repetition time TR was 2000ms, the echo time TE was 250ms and the frequency of excitation (the number of signal averages) was 2, the time required for signal acquisition was 17 minutes.
Conventionally, as a method of obtaining an echo corresponding to a spin-spin relaxation time T2 within a short period of time, the so-called gradient echo method has been known. For reference, Journal of Magnetic Resonance 62, 12-18 (1985) and magnetic Resonance in Medicine 7, 35-42 (1988) should be noted.